numeric.c: split NUM_STEP_GET_INF

* numeric.c (NUM_STEP_GET_INF): split from NUM_STEP_SCAN_ARGS(), since
  inf is not used in num_step_size().

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42829 b2dd03c8-39d4-4d8f-98ff-823fe69b080e